Wavelength division multiplexing (WDM) is used to increase the communication bandwidth or the number of communication channels in optical communications. A number of optical signals carried by light having different wavelengths are input and propagating in a single optical fiber. A WDM multiplexer (MUX) is used to combine a number of optical signals carried by light having different wavelengths into a fiber. To detect each signal, the combined light exiting from the fiber is decomposed into its components having different wavelengths using a WDM demultiplexer (DeMUX). Each component corresponds to an optical signal. Typically, the optical signals decomposed by the DeMUX form an optical spot array on a cross-section plane. The pitch of the array is determined by the DeMUX. The optical signals are detected by an array of photo diodes (PD) in a receiver optical sub-assembly (ROSA).
Similarly, an array of laser diodes (LD) in a transmitter optical sub-assembly (TOSA) emits optical signals. Typically, the optical signals emitted by the LD array in the TOSA form an optical spot array on a cross-section plane. The optical signals are combined by a WDM MUX into an optical fiber. Accordingly, the optical spot array pitch of the passive WDM MUX or DeMUX must match the optical spot array pitch of the active LD or PD array.
The pitch of an active LD or PD array may be 3.05 mm, which is the diameter of the transistor outline (TO) can for packaging LD or PD. On the other hand, the pitch of an active LD or PD array may be 0.25 mm for an integrated LD or PD. The pitch of an active LD and PD array may be any number, which is determined by the manufacture of the device. Similarly, the pitch of a passive WDM MUX and DeMUX may be any number as well, which is determined by its manufacturer. Accordingly, an optical spot array pitch compressor to match the pitch of a passive WDM MUX or DeMUX with the pitch of an active LD or PD array is required. Furthermore, the optical spot array pitch compressor must be able to provide a varying compressor ratio. Especially, when the passive WDM MUX and DeMUX and the active LD and PD array are not made based on the same specification, or are made by different manufacturers. It is appreciated that it is almost impossible to compress the optical spot pitch of a passive WDM MUX or DeMUX to as small as 0.25 mm pitch using traditional free space optics.
U.S. Pat. No. 7,023,620 to Sandberg et al. discloses a device 100 to provide beam pitch compression using a group of mirror as shown in FIG. 1. Four beams 102, 104, 106, and 108 having wavelengths λ1, λ2, λ3, and λ4, respectively, are separated into a first group including beams 102 and 104, and a second group including beams 106 and 108. The first group including beams 102 and 104 is reflected 90° by a first mirror 110. The second group including beams 106 and 108 is reflected 90° by a second mirror 112. Second mirror 112 has a hole or window to allow beam 104 passing through. If there are more than four beams, second mirror 112 must have a periodical structure mirror-window-mirror-window to reflect beams of the second group and to transmit beams of the first group. Device 100 will change the order of beams 102, 104, 106, and 108 to a new order of beams 102, 106, 104, and 108. The special structure of second mirror 112 will increase the cost. Device 100 will have a fix compression ratio instead of a varying compression ratio.
U.S. Pat. No. 4,627,690 to Fantone discloses an anamorphic prism 200 for beam compression as shown in FIG. 2. An incident beam 202 enters anamorphic prism 200 normally from a right angle surface 204. After having two total internal reflections (TIR) at an inclined surfaces 206 and a flat surface 208, incident beam 202 is refracted from inclined surface 206 to the air becoming an output beam 210. Incident beam 202 originally has a beam diameter Win. Output beam 210 has a compressed beam diameter Wout. The compression ratio is Win/Wout. Anamorphic prism 200 requires the following conditions be satisfied.
                              n          =                                    cos              ⁢                                                          ⁢              α                                      cos              ⁢                                                          ⁢              3              ⁢                                                          ⁢              α                                      ,                            Equation        ⁢                                  ⁢                  (          1          )                                                  η          =                                                    W                in                                            W                out                                      =                          2              +                              1                n                                                    ,                            Equation        ⁢                                  ⁢                  (          2          )                    where n is the refractive index of anamorphic prism 200, a is an apex angle 212 of anamorphic prism 200, and η is the compression ratio.
Apex angle 212, α, is determined in a range of 17° to 19° by the refractive index n. Compression ratio η, which is in a range of 2 to 3, is also determined by the refractive index n. Due to the small apex angle)(17°˜19°, the prism must have a long length L 214 to fully transmit the beam through the prism. Furthermore, inclined surface 206 includes an area of TIR 216, which is not coated, and an area of refraction 218, which is anti-reflection (AR) coated. To separate two areas 216 and 218 in an AR coating process, the prism may not be small. Anamorphic prism 200 has a fix compression ratio instead of a varying compression ratio.
Accordingly, an optical spot array pitch compressor to match the pitch of a WDM MUX or DeMUX with the pitch of a LD or PD array, which is simple, small, low cost, and capable of providing a varying compression ratio, is desired.
Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention.